To make the croutons, preheat the oven to 350˚ F. Add the bread cubes to a medium bowl and toss with the melted butter, olive oil, and mustard until evenly coated. Spread the bread cubes out in an even layer on a baking sheet and sprinkle with the salt. Bake for 10-15 minutes or until crunchy.

Meanwhile, melt the butter in a large pot or Dutch oven over medium-high heat. Add the shallots and onion and cook, stirring occasionally, until slightly softened, about 4-5 minutes. Season with salt, then mix in the potato, garlic and vegetable broth. Cover the pot and bring to a boil. Lower the heat to a simmer, keep covered, and let cook for about 8 minutes or until the potatoes are tender. Add the cauliflower and cook about 5-6 minutes more, until the cauliflower is tender. Stir in the cheese and mustard.

Puree with an immersion blender (or a food processor, vented to allow the steam to escape). Mix in additional water or broth if the soup is too thick. Season with additional salt to taste. Serve warm topped with additional shredded cheese and the croutons.